A number of methods have been reported for producing tin oxide and related oxide materials. Powders have been produced and studied for a number of years. For certain applications, however, the advantages of films have made film deposition the fabrication method of choice.
The richness of the chemical, electronic and mechanical properties of oxide films has made them useful in a wide variety of applications. The majority of the technological films currently available are polycrystalline. Polycrystalline films result from processes which do not rigorously control the microstructure of the film during deposition. These films are textured and have distributions of varying crystalline structures, sizes, and orientation and a large number of grain boundaries. Additionally, while polycrystalline films of oxide having extended lateral dimension (&gt;1 cm.sup.2) have been obtained, highly ordered oxides have been generally limited to single crystals, the dimensions of which for many oxides are are typically less than about 1 cm.sup.2.
Nagano in "Chemical Vapor Deposition of Tin Oxide Thin Films on Rutile Single Crystals", J. Crystal Growth, 67, 639-644 (1989), teaches the deposition of thin films of tin oxide on the surface of titanium oxide by chemical vapor deposition. Kane, et al., in "Chemical Vapor Deposition of Antimony Doped Tin Oxide Film Formed from Dibutyl Tin Diacetate", J. Electrochem. Soc.: Solid State Science and Technology, volume 123, Number 2, February, 1976, teaches a chemical vapor deposition process for preparing transparent conducting layers of antimony doped tin oxide utilizing dibutyl tin diacetate, antimony pentachloride, oxygen, water and nitrogen as a carrier gas at a substrate temperature of 400.degree. to 550.degree. C. This method was designed to fulfill a need for more highly conducting coatings than those obtainable without doping, while still possessing optical transmission in excess of 80% throughout the visible spectrum.
In "Growth of Highly Oriented Tin Oxide Thin Films by Laser Evaporation Deposition", Appl. Phys. Lett., 57, 29 Oct. 1990, Dai et al. disclose conducting thin films of tin oxides which were prepared by the laser evaporation of a pressed polycrystalline undoped powder tin oxide onto unheated substrates. Kuriki et al. in "Gas sensing Characteristics of Reactively Sputtered SnO.sub.2 Films", Electronics and Communications in Japan, Part 2, Vol. 70, No. 4, 1987, disclose gas sensors which are composed of polycrystalline SnO.sub.2 films. In many cases imperfections in these oxides such as grain boundaries, surface steps, and oxygen vacancy defects set limits on the functionality of the film.
Nearly perfect (pure), stoichiometric, structurally ordered in both short and long range, smooth, crystal-like tin oxide films formed by heteroepitaxial growth would be expected to exhibit more uniform properties and interactions when presented to technological environments and stimuli, as for example gases, photons, and electrons. With only one crystalline orientation throughout and no defect sites the adsorption kinetics would be coherent across the film. Removing structural defects (large ones such as grain boundaries, or small ones such as individual vacancy sites), would produce a much more uniform optical density by removing random scattering sites for transparent conductors. Near perfection in such films could also be expected to yield higher mobilities in electronic transport and when the perfection extends out to the surface, it could allow unique control of frictional coefficients.
While the advantages of producing high quality completely epitaxial films of non-oxide materials has been widely recognized in the semiconductor industry, efforts to produce films at equivalent quality for oxide materials have largely been confined to research on the high temperature superconductor materials.
Chang et al. in "Ceramic Epitaxial Films and Multilayers Prepared by MOCVD", Proceedings of the 7th CIMTEC World Ceramics Congress, Montecatini Terme, Italy, Jun. 24-30, 1988 and in "Epitaxy of TiO.sub.2 Thin Film on Sapphire by MOCVD", MRS Spring Meeting Symposium J, "Thin Film Structures and Phase Stability", San Francisco, Calif. Apr. 16-21, 1990, report the production of epitaxial titanium oxide films using MOCVD. In "Epitaxy of TiO.sub.2 Thin Film on Sapphire by MOCVD" supra, the growth of epitaxial titanium oxide films on sapphire substrates by thermally decomposing titanium isopropoxide in the presence of oxygen in a cold wall low pressure MOCVD system is disclosed. In "Ceramic Epitaxial Films and Multilayers Prepared by MOCVD", supra, epitaxial titanium oxide and vanadium oxide films on sapphire substrates are produced using the MOCVD process for growth of the films in single and multilayer configurations. Titanium oxide and aluminum oxide films were deposited using molecular beam epitaxial methods in "Titanium Oxide Aluminum Oxide Multilayers Ceramic Thin Films", Alexander et al., J.Am. Ceram. Soc., vol. 73( 6), 1990. In "Epitaxial Thin Films of ZnO on Cds and Sapphire", J. Vacuum Sci. and Tech., vol. 6, no. 1, 1968, Rozgonyi et al., discuss epitaxial films of ZnO, a non-rutile structured oxide, produced by sputtering on sapphire. Shiosaki et al, in "High Rate Epitaxial Growth of ZnO Films on Sapphire by Planar Magnetron Sputtering System", J. Crystal Growth, 45 (1978) 346-349 production of epitaxial films of ZnO were obtained on sapphire.
Additionally, the development of chemical sensors based on thin film technology is a rapidly growing area in chemical sensing. Thin film sensors offer the potential advantages of lower cost, greater sensitivity, less drift, and the achievement of selectivity by integrating different sensors on a single wafer for using semi-conductor microfabrication techniques. Key to development of such a technology is an understanding of what film characteristics are important for sensing and how to optimize processing to enhance those characteristics.